It is long-standing state of the art to provide at load changeover switches of tapped transformers two permanent contacts which in stationary operation conduct the permanent current of the is respectively connected side A or B of the load changeover switch and thus unload the actual switching contacts of the load changeover switch. At the start of each changeover process, i.e. each actuation of the load changeover switch, the permanent main contact previously conducting the current opens first and the current is commutated to the switch contacts of the load changeover switch; after the conclusion of the entire load changeover process the other permanent main contact then closes and takes over the permanent current again.
A constructive embodiment of a load changeover switch according to category with permanent main contacts switching electrically powerlessly for each phase is known from DE 103 12 176 B3, wherein each of the permanent main contact pairs has a common electrically conductive permanent main contact which is pivotable into two different end settings and thus alternatively bridges over the respective permanent main contact pairs in stationary operation. Arranged for this purpose in the interior of the load changeover switch is a centrally extending switching shaft by which the movable electrically conductive permanent main contact constructed as a bridging contact is actuatable. Each permanent main contact pair consists of a first permanent main contact and a second permanent main contact, which are electrically insulated from one another. One of these first permanent main contacts is electrically connected with a side A and the other one of these permanent main contacts is electrically connected with the other side B of the load changeover switch. The two second permanent main contacts are permanently connected with a common load shunt of the load changeover switch, since the run-up surfaces of the pivotable permanent main contact are spherically dimensioned in such a manner that on pivoting of the permanent main contact about its bearing they remain constantly in contact with the two second permanent main contacts of the two permanent main contact pairs, which are electrically connected with the load shunt; this takes place in the form of a sliding or rolling motion. Thus, there is thereby a permanent electrical connection of the movable permanent main contact with the load shunt regardless of the setting of the movable permanent main contact. By contrast thereto, the two contact surfaces, which are respectively arranged thereunder, of the movable permanent main contact are so constructed that they come into contact with the respectively corresponding fixed first permanent main contacts only on pivoting of the movable permanent main contact into one of its two end positions.
The actuation of the movable permanent main contact takes place through an actuating strap which has a guide in the form of a slot and is articulated by fastening means. Running in the guide is an actuating pin of a deflecting crank which in turn is mounted to be rotatable about a fulcrum. The deflecting crank has at the free and thereof a profile consisting of roller engagements as well as a blocking profile disposed therebetween.
Thus, in DE 103 12 176 B3 two crank drives connected one behind the other are functionally necessary for the permanent main contact actuation, which drives due to the construction can be subject to tolerances and in addition manage guidance of the deflecting crank during the actual changeover process purely by way of an only small circular sector of a frictional blocking profile provided at the end face of the drive disk.